In general, a particle beam therapy system is provided with a beam generation apparatus that generates a charged particle beam, an accelerator that is connected with the beam generation apparatus and accelerates a generated charged particle beam, a beam transport system that transports a charged particle beam that is accelerated by the accelerator so as to gain predetermined energy and then emitted, and a particle beam irradiation apparatus, disposed at the downstream side of the beam transport system, for irradiating a charged particle beam onto an irradiation subject. Particle beam irradiation apparatuses are roughly divided into apparatuses utilizing a broad irradiation method in which a charged particle beam is enlarged in a dispersion manner by a scatterer, and the shape of the enlarged charged particle beam is made to coincide with the shape of an irradiation subject in order to form an irradiation field; and apparatuses utilizing a scanning irradiation method (the spot-scanning method, the raster-scanning method, and the like) in which an irradiation field is formed by performing scanning with a thin, pencil-like beam in such a way that the scanning area coincides with the shape of an irradiation subject.
In the broad irradiation method, an irradiation field that coincides with the shape of a diseased site is formed by use of a collimator or a bolus. The broad irradiation method is a most universally utilized and superior irradiation method where an irradiation field that coincides with the shape of a diseased site is formed so as to prevent unnecessary irradiation onto a normal tissue. However, it is required to create a bolus for each patient or to change the shape of a collimator in accordance with a diseased site.
In contrast, the scanning irradiation method is a high-flexibility irradiation method where, for example, neither collimator nor bolus is required. However, because these components for preventing irradiation onto not a diseased site but a normal tissue are not utilized, there is required a positional accuracy of beam irradiation that is the same as or higher than that of the broad irradiation method.
The size of a beam transported from an accelerator is several millimeters, in general; in contrast, in the case of medicine, the irradiation coverage of a charged particle beam needs to be several tens of centimeters square or larger. In order to obtain a wide irradiation field by use of a thin charged particle beam, the foregoing scanning irradiation method is utilized.
Patent Document 1 discloses the following invention whose objective is to provide a rotating gantry that enlarges the irradiation coverage in a direction that is parallel to the deflection plane, while keeping the intensity of the scanning electromagnet as large as the intensity of a conventional one. In the invention disclosed in Patent Document 1, the deflection electromagnet and the irradiation field moving electromagnet at the upstream side of the scanning electromagnet change the downstream beam position; the scanning electromagnet is moved to the beam positions a and b; an irradiatable area A is irradiated and, after that, the charged particle beam is made to pass through the other position b at the downstream side of the deflection electromagnet; then, the scanning electromagnet is moved to the beam position b and an area B is irradiated; in such a manner, the irradiation coverage is enlarged in each of the areas A and B.